In semi-conductor components, in particular for example in corresponding integrated (analog and/or digital) computing circuits, for example micro-processors and/or micro-controllers etc. and semi-conductor memory components, as well as other electrical circuits and/or signal-processing systems, for example filter circuits, digital-analog converters, amplifiers, regulators, etc. the problem that often needs to be solved is the subtraction of corresponding electrical signals from each other with a high degree of accuracy.
The electrical signals to be subtracted from each other could for example be generated by sensors and/or by corresponding circuit configurations, etc.
Relatively simply constructed state of the art assemblies, for instance a simple current node are available, with which electrical signals can be subtracted from each other. A common disadvantage here is among others often the fact that distortions and/or non-linearities are not able to be corrected with such simple devices.
In FIG. 1 an example of a conventional simple circuit device 1 for the subtraction of electrical signals (here: of currents I_1 and I_2 present on corresponding lines 5, 6) is shown.
This comprises two n-channel field effect transistors 2, 3—constituting a current mirroring device—, and an operational amplifier 4.
As is apparent from FIG. 1, the gate of the n-channel field effect transistor 2 is connected via a line 10 with the gate of the n-channel field effect transistor 3, and is connected via a line 7 with the above line 5, and back-connected via a line 8 with the drain of the n-channel field effect transistor 2.
The source of the n-channel field effect transistor 2 is connected to ground via a line 9.
In correspondingly similar fashion the source of the n-channel field effect transistor 3 is connected to ground (here: via a line 11).
As is further apparent from FIG. 1, the n-channel field effect transistor 3 (more accurately: the drain of the n-channel field effect transistor 3) can be connected via a line 13 with a first input of the operational amplifier 4, and the n-channel field effect transistor 2 (more accurately: the drain of the n-channel field effect transistor 2) can be connected via a line 14 with a second input of the operational amplifier 4.
The output of the operational amplifier 4 is back connected via a line 12 with the (first) operational amplifier-input.
With the help of the operational amplifier 4 it is attempted to regulate the potential at the drain of the n-channel field effect transistor 3 (i.e. the potential at a Point B of the circuit device 1 illustrated in FIG. 1) to the potential at the drain of the n-channel field effect transistor 2 (i.e. the potential at a Point A of the circuit device 1 illustrated in FIG. 1).
The purpose of this measure is the elimination of subtraction faults that can be ascribed to early voltages at the n-channel field effect transistors 2, 3 (and thereby of a major distortion component of subtraction faults) from the differential current I_diff made available by the circuit device 1 (detectable at line 13).
One problem is inter alia that the variable gain amplification of the operational amplifier 4—and/or of other conventional variable gain amplifier circuits—may be too small for the above purpose. In particular a p-channel field effect transistor 15 provided in the operational amplifier 4 may have insufficient regulatory scope for particular applications (in particular for example due to the fact that the threshold potential in n-channel field effect transistors is generally lower than that in p-channel field effect transistors). For an adequate regulatory scope the gate of the p-channel field effect transistor 15 would have to be moved towards negative voltages (which is not permissible, due to the corresponding voltage lift required).
A further disadvantage of the circuit device 1 shown in FIG. 1 to be mentioned is for example the fact that the threshold voltages of an n and a p-channel field effect transistor operate against each other as a result of the diode characteristics of the n-channel field effect transistor 2, and of the p-channel field effect transistor 15 provided in the operational amplifier 4 and functioning as a control transistor, which can be a considerable disadvantage regarding the robustness of the circuit device 1 against process and/or manufacturing inaccuracies.